1. Field of the Invention
The present invention relates to an alkali silicate aqueous solution having a high molar ratio (SiO2/(A2O+B) (A: alkali metal, B: NH3)), a high silicon content and a high anionic activity, a process for preparing the solution and uses of the solution.
2. Description of Related Art
An alkali silicate aqueous solution called water glass contains relatively large amounts of alkali ions in order to maintain the solution state, and therefore the molar ratio of silicon to an alkali (SiO2/(A2O+B) (A: alkali metal, B: NH3)) is usually less than 4. Although silicic acid ions and alkali ions are contained in the solution, the negative charge quantity is small, so that the anionic activity is low and the ζ potential that is an indication of the anionic activity is in the range of −14 MV or less but more than −40 MV.
On the other hand, primary particles called silicic acid sol or colloidal silica have neither internal surface area nor crystalline portion, and they are dispersed in an alkaline medium. The alkali reacts with the silica surface to form negative charge on the silica surface, and because the silica particles are negatively charged, they are stabilized by virtue of the repulsion of the negative charge between the particles. On the surface of the silica colloidal substance, however, many silanol groups (Si—OH) are present in addition to the silicic acid anions which form the negative charge. Hence, the negative charge quantity is small and the ζ potential is in the range of −25 to −38 MV.
By the dealkalization of water glass, a silicic acid sol is obtained, but a stable intermediate between the water glass and the silicic acid sol has not been obtained. The reason is that since the molar ratio is raised with the progress of dealkalization, the water glass cannot maintain the solution state thereof. When the molar ratio becomes 4.2 or more, precipitation of silica takes place and the water glass cannot maintain the solution state.
If a high-molar ratio alkali silicate aqueous solution having solution-like characteristics such as those of water glass and having a high molar ratio and a high SiO2 concentration similar to the silicic acid sol is obtained, development of various uses can be expected.
That is to say, there has been desired a process to raise the molar ratio, the activity and the SiO2 concentration with retaining the solution-like characteristics of the water glass.
However, the molar ratio cannot be raised by only concentrating the water glass through evaporation. For example, if a water glass product having the highest molar ratio 4.0 is concentrated to a SiO2 concentration of 30% by weight, the product gels completely.
On the other hand, concentration of colloidal silica by ultrafiltration is carried out (see U.S. Pat. No. 3,969,266, U.K. Patent No. 1,148,950, Japanese Patent Laid-Open Publication No. 15022/1983, etc.). If the colloidal silica is particle-grown silica, it can be satisfactorily concentrated by the ultrafiltation. In case of the water glass, however, the amount of the low-molecular weight component such as ion is large and the yield obtained by the ultrafiltration is low. Moreover, because of large loss of ions, the anionic activity inherent in the water glass is lost.
It is an object of the present invention to provide an alkali silicate aqueous solution having intermediate characteristics between water glass and colloidal silica and having a high molar ratio (SiO2/(A2O+B) (A: alkali metal, B: NH3)), a high silicon content and a high anionic activity, a process for preparing the solution and uses of the solution.